1. Field of the Invention
The invention relates to a semiconductor light emitting device, and particularly relates to a semiconductor light emitting device that can include a modified electrode structure.
2. Description of the Related Art
An opaque pad electrode whose rear surface is wire bonded can be employed in conventional type semiconductor light emitting devices. Even if light is emitted, the light is shielded or absorbed by the pad electrode. Light emissions cannot be efficiently produced on the rear surface area and the luminous efficiency drops with respect to the applied electrical power.
In order to solve this problem, a device called a current confined path type light emitting diode can be used. This type of LED is provided with a pad electrode locally formed on the topmost layer of semiconductor layers and a linear electrode with an approximate mesh shape part of which makes contact with the pad electrode. Furthermore, a Schottky contact is formed between the pad electrode and the topmost layer of semiconductor layers, thereby preventing the leakage of current and the emission of light at areas covered by the pad electrode as well as preventing decreases in the quantity of light emitted with respect to the applied electrical power.
FIG. 1 and FIG. 2 show examples of the configuration of this type of conventional current confined path type light emitting diode 90 (for example, see Japanese Patent Laid-Open Publication No. 2004-296979, which is hereby incorporate in its entirety by reference). As shown in the cross section in FIG. 1, this light emitting diode 90 includes a P-type semiconductor layer 92, an active layer 93, an N-type semiconductor layer 94, and a topmost semiconductor layer 95 formed of n-AlGaInP.
As shown in FIG. 2, on the topmost semiconductor layer 95 a linear electrode 96 is provided made of a metal such as An/Sn/Ni. The linear electrode 96 has a shape like a spider web that can evenly supply electrical current over a wide range of the topmost semiconductor layer 95. The topmost semiconductor layer 95 and the linear electrode 96 are thermally alloyed to form an ohmic contact therebetween.
An electrode used for a pad 97 is formed at the center of the linear electrode 96 so as to make contact with both the topmost semiconductor layer 95 and the linear electrode 96. Metal material, such as Ti/Au/Pt, that has work functions larger than the electron affinity of the topmost semiconductor layer 95 is selected for the electrode used for a pad 97 at this time. Then, a bonding wire 98 is connected to the electrode used for a pad 97 to allow power to be supplied from an external source.
According to this configuration, an ohmic contact is formed between the topmost semiconductor layer 95 and the linear electrode 96. An ohmic contact is also formed between the linear electrode 96 and the electrode used for a pad 97, and only a Schottky contact is formed between the topmost semiconductor layer 95 and the electrode used for a pad 97.
Therefore, the electrical power supplied to the bonding wire 98 is transmitted to the topmost semiconductor layer 95 from the electrode used for a pad 97 through the linear electrode 96 and electrical current is not supplied to the topmost semiconductor layer 95 from the electrode used for a pad 97 (Schottky connected). Because of this, light does not irradiate from the area under the electrode used for a pad 97 and there is no ineffective electrical power consumed.
In the configuration of the conventional current confined path type light emitting diode 90 described above, the irradiation of light from the area under the electrode used for a pad 97 can surely be prevented, although a problem occurs in which a metal such as Ge or Zn, which is added to the linear electrode 96 in order to improve the ohmic contact between the topmost semiconductor layer 95 and the linear electrode 96, deposits on the electrode used for a pad 97 during the thermal alloying, thereby weakening the bonding strength of the bonding wire 98.
When the linear electrode 96 is laid out, the position of the linear electrode 96 should be taken into consideration because the linear electrode 96 may also shield the light irradiation. To cope with this problem, it is possible to thinly form the electrode in a range in which a sufficient amount of electrical power will be provided to the LED chip 91. However, a phenomenon easily occurs in which Ge or Zn, which is added to the linear electrode 96 as described above, concentrate at one area during the thermal alloying and, for example, the resistance value of the linear electrode 96 thereby increases. Due to this, there is a limit of approximately 5 μm on the width, and the electrode cannot be thinly formed. Consequently, although the configuration is complicated, a problem also occurs wherein the light gathering efficiency is not improved.